9. The future

Perception is one of the best understood topics in psychology and yet there is still no universal agreement as to how we should understand the purpose or objective of perceptual processes. Should it be to explain how we see, hear, smell, taste, and touch—our subjective experiences—or should it be to explain how sensory information guides and controls action? ‘The future’ considers to what extent our perceptual processes are cognitively penetrable, that is, affected by higher-level processes of attention, expectations, emotions, and knowledge. It also explains the important consequences of ecological validity for the kinds of experiments we use to study perception.

4. Motion perception

The ability to detect motion is one of the most important properties of our visual system and the visual systems of nearly every other species. Motion perception is not just important for detecting the movement of objects—both for catching prey and for avoiding predators—but it is also important for providing information about the 3-D structure of the world, for maintaining balance, determining our direction of heading, segregating the scene and breaking camouflage, and judging time-to-contact with other objects in the world. ‘Motion perception’ describes the spatio-temporal process of motion perception and the perceptual effects that tell us something about the characteristics of the motion system: apparent motion, the motion after-effect, and induced motion.

1. What is perception?

The word ‘perception’ can be used in two different ways. It can refer to our experience of seeing, hearing, touching, tasting, and smelling objects and individuals around us. It can also refer to the processes that allow us to extract information from the patterns of energy that impinge on our sense organs. Thinking about perception as a set of processes has the advantage that it includes situations where there is no subjective experience. ‘What is perception?’ explains that sometimes our perceptual systems can be fooled and we experience illusions. Is this because of past experience and our knowledge of the world, or is it that we are not extracting the information in the patterns of energy reaching our senses?

8. The physiology and anatomy of the visual system

‘The physiology and anatomy of the visual system’ describes what we have learned from neurophysiology and anatomy over the past eighty years and what this tells us about the meaning of the circuits involved in visual information processing. It explains how psychologists and physiologists use the terms ‘mechanism’ and ‘process’. For physiologists, a mechanism is linked to the actions of individual neurons, neural pathways, and the ways in which the neurons are connected up. For psychologists, the term is typically used to describe the processes the neural circuits may carry out. The human retina is described with explanations of lateral inhibition, receptive fields, and feature detectors as well as the visual cortex and different visual pathways.

7. Delusions about illusions

The term illusion is used to describe situations where we make mistakes and perceive the surrounding world incorrectly. But what is an illusion? Richard Gregory described illusions as ‘departures from reality’ and this fits with our everyday idea that illusions represent situations where what we perceive does not correspond to some physical characteristic of the particular scene. ‘Delusions about illusions’ considers illusions that have a physiological basis and those with a cognitive basis. It seems very likely that our perceptual systems have evolved to extract the invariant characteristics of the world—the things that don’t change—rather than requiring mechanisms to ‘correct’ for things that do change.

2. Perceptual theories—direct, indirect, and computational

‘Perceptual theories—direct, indirect, and computational’ considers three different conceptions of what it means to perceive and the processes involved in each theory. The origins of indirect or constructivist theory can be traced back to Hermann von Helmholtz in the 19th century, who emphasized the importance of experience in shaping our perceptual abilities. It was assumed that the primary purpose of perception was to create subjective experiences. The American psychologist James Gibson first suggested a direct theory—that the primary role of perceptual processes was to guide action. Since the 1960s, there have been many attempts to model the perceptual processes using computer algorithms, with David Marr at MIT being the most influential figure.

6. Perception and action

Perceptual processes have evolved to allow us to act appropriately. ‘Perception and action’ explains that the strong link between perception and action has been revealed in a variety of relatively low-level tasks such as helping us to maintain balance; providing information about our movements within the world (self-motion); allowing us to determine our direction of travel (heading); and helping us to estimate the time before we reach objects in the surrounding world (time-to-contact). James Gibson has suggested that the purpose of our perceptual systems is not to identify objects as objects, but rather to extract information about what we might do with those objects and how we might respond to events in the world.

3. Lightness and colour

‘Lightness and colour’ considers visual perception and attempts to identify some of the characteristics of the patterns of light reaching our eyes that might contribute to the information we need for survival. It discusses trichromatic vision in humans; the problem of metamerism; and the implications trichromacy has for the ability to describe and differentiate colours, including intensity, hue, and saturation. The pick-up of particular perceptual characteristics—the lightness and colour of the surfaces that surround us—should not be thought of as ends in themselves, but rather as a contribution to a perceptual system that has evolved to allow us to act successfully in a particular ecological niche.

5. Perception of a 3-D world

The ability to perceive the 3-D world has often been regarded as a task that poses particular problems for the visual system. However, ‘Perception of a 3-D world’ argues that we are particularly fortunate because there are multiple sources of information to tell us about the different aspects of the 3-D structure of objects. It discusses three of these sources of information—perspective, occlusion, and shading—and then explains motion parallax, optic flow, binocular stereopsis, eye vergence and depth constancy, vertical disparities and differential perspective, and primary and secondary depth cues. The effectiveness of these different sources of 3-D information is considered along with how they are all brought together.